galaxy, any of the systems of stars and interstellar matter that make up the universe. Many such assemblages are so enormous that they contain hundreds of billions of stars.
Nature has provided an immensely varied array of galaxies, ranging from faint, diffuse dwarf objects to brilliant spiral-shaped giants. Virtually all galaxies appear to have been formed soon after the universe began, and they pervade space, even into the depths of the farthest reaches penetrated by powerful modern telescopes. Galaxies usually exist in clusters, some of which in turn are grouped into larger clusters that measure hundreds of millions of light-years across. (A light-year is the distance traversed by light in one year, traveling at a velocity of 300,000 km per second [km/sec], or 650,000,000 miles per hour.) These so-called superclusters are separated by nearly empty voids, and this causes the gross structure of the universe to look somewhat like a network of sheets and chains of galaxies.
Galaxies differ from one another in shape, with variations resulting from the way in which the systems were formed and subsequently evolved. Galaxies are extremely varied not only in structure but also in the amount of activity observed. Some are the sites of vigorous star formation, with its attendant glowing gas and clouds of dust and molecular complexes. Others, by contrast, are quiescent, having long ago ceased to form new stars. Perhaps the most conspicuous activity in galaxies occurs in their nuclei, where evidence suggests that in many cases supermassive objects—probably black holes—lurk. These central black holes apparently formed several billion years ago; they are now observed forming in galaxies at large distances (and, therefore, because of the time it takes light to travel to Earth, at times in the far distant past) as brilliant objects called quasars.
The existence of galaxies was not recognized until the early 20th century. Since then, however, galaxies have become one of the focal points of astronomical investigation. The notable developments and achievements in the study of galaxies are surveyed here. Included in the discussion are the external galaxies (i.e., those lying outside the Milky Way Galaxy, the local galaxy to which the Sun and Earth belong), their distribution in clusters and superclusters, and the evolution of galaxies and quasars. For further details on the Milky Way Galaxy, see Milky Way Galaxy. For specifics about the components of galaxies, see star and nebula.
Galaxy Types of details:
What Kinds of Galaxies Are There? Astronomers classify galaxies into three major categories: elliptical, spiral and irregular. These galaxies span a wide range of sizes, from dwarf galaxies containing as few as 100 million stars to giant galaxies with more than a trillion stars.
Spiral galaxy
The first type of galaxies—spiral galaxies—can be recognized by their wide, flat disks of rotating gas and dust. Some spirals have wide flung arms like the above image of M51, while others have spirals that are more tightly bound.
Fun fact: our very own Milky Way is a spiral galaxy, and it is rotating at 168 miles per second. Now that’s fast!
Elliptical galaxy
Elliptical galaxies are given their name because of their oblong shape and lack of overall structure. Some are nearly spherical, while others are shaped more like a cigar. They can range in size from just a few light years across to being larger than our Milky Way.
Another fun fact: while it is not entirely clear how they form, some scientists propose that elliptical galaxies form as the result of galactic collisions!
Peculiar galaxy
Speaking of galactic collisions, peculiar galaxies are almost always the result of one!
Galactic collisions aren’t as action packed as you might imagine.
Since galaxies are mostly comprised of empty space, it’s unlikely that any two given stars will collide during the event of a galactic collision.
It is believed that peculiar galaxies compose 5 to 10 percent of all known galaxies. So, it’s safe to say that galactic collisions are a common occurrence in our own universe!
Irregular galaxy
Irregular galaxies are galaxies that don’t fall under any of the three galaxy types mentioned above. These galaxies tend to be small, dwarf galaxies that lack any distinguishable shape. Many of these galaxies are companions or satellites to larger galaxies.
A final fun fact: The Milky Way has dozens of irregular satellite galaxies. Arguably the most famous one is the Large Magellanic Cloud, shown above. *****
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